A battery is an electrochemical device comprising an anode, a cathode, a separator sandwiched between the anode and cathode, and an electrolyte wetting the separator and in ionic communication between the anode and the cathode. Various chemistries in which the electrochemical potential between various materials is used to generate electricity have been studied and commercially implemented. See, in general: Besenhard, J. O., Ed., Handbook of Battery Materials, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999; and Linden, D., Ed., Handbook of Batteries, Second Edition, McGraw Hill Inc., New York, N.Y., 1995, both of which are incorporated herein by reference.
The separator is a critical element of the battery. It is used as a spacer between the anode and cathode to prevent electronic contact of the anode and the cathode. It must be sufficiently permeable to allow ion transport between the anode and the cathode, but must have pores that will block the migration of particles that separate from the electrodes. It must have sufficient strength and/or tortuosity to prevent dendrites from growing through the separator and causing a short circuit within the battery. It must also have sufficient strength to withstand the battery manufacturing process. Such strength manifests itself, in one instance, as puncture strength, the ability withstand being pierced by the rough surfaces of the electrodes during battery manufacture. Such strength also manifests itself, in another instance, as tensile strength, the ability to withstand the tight winding required to obtain the battery's physical structure. The separator must also be thin, so as not to take up too much space within the cell. Accordingly, there is a need for tough, strong, thin, and permeable structures for use as separators in batteries.
One structure for obtaining such qualities is a nonwoven material or fabric. See, for example, Besenhard, Ibid., page 288 and U.S. Pat. Nos. 5,605,549; 5,962,161; 6,120,939; and U.S. application Ser. No. 09/745,026 filed Dec. 20, 2000, each of which is incorporated herein by reference. Nonwoven materials encompass a broad class of fibrous structures. Nonwoven material, in its most common usage, includes fibrous structures made by such processes as dry, wet, or air laying, needlepunching, spunbond or melt blown processes, and hydroentanglement. Melt blown nonwovens are different from spunbond nonwovens. Spunbond nonwovens are filaments, and then laying those filaments onto a continuous belt, that result in a nonwoven with long fiber lengths. Melt blown nonwovens are formed by extruding molten polymer through a die then attenuating and breaking the resulting filaments with hot, high-velocity air or steam, that results in a nonwoven with short fiber lengths. As battery separators, melt blown nonwovens are perceived as having some adequately sized pores within the distribution of pore sizes, but having to broad a distribution of pore sizes (thus having to many inadequately, too large, sized pores and having insufficient tensile and puncture strength, both of which limit their ability to be used as effective battery separators.
Electrolytes, used in the most widely sold batteries, may be aqueous (polar) or organic (nonpolar) solutions. Since, batteries are most often assembled without electrolyte, and are later activated by the addition of the electrolyte, the separator must be wettable by the electrolyte. “Wettability” or “wet out” refers to the ability to cause a liquid to penetrate more easily into, or spread over, the surface of another material. Hydrophilic refers to the ability to “wet out” an aqueous based liquid. Hydrophobic refers to the inability to “wet out” an aqueous based liquid. Accordingly, nonwovens, which are, most often, made of polyolefins, will not wet out aqueous based electrolytes because polyolefins are inherently hydrophobic. Therefore, such nonwovens must be made hydrophilic so that the aqueous electrolyte will wet out the separator.
Accordingly, there is a need for a wettable, melt blown nonwoven separator having sufficient mechanical properties and sufficiently small pores for use as a separator in a battery.